The invention relates to techniques for making improved noncontact temperature measurements of a semiconductor substrate by enhancing the effective emissivity of the substrate and by correcting measurements of substrate temperature.
In many semiconductor device manufacturing processes, the required high levels of device performance, yield, and process repeatability can only be achieved if the temperature of a substrate (e.g., a semiconductor wafer) is tightly controlled during processing. To achieve that level of control, it is often necessary to measure the substrate temperature in real time and in situ, so that any unexpected temperature variations can be immediately detected and corrected for.
For example, consider rapid thermal processing (RTP), which is used for several different fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). In the particular application of CMOS gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are critical parameters that influence the overall device performance and fabrication yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 .ANG. thick and for which thickness uniformity must be held within .+-.2 .ANG.. This level of uniformity requires that temperature variations across the substrate during high temperature processing cannot exceed a few .degree.C.
The wafer itself often cannot tolerate even small temperature differentials during high temperature processing. If the temperature difference is allowed to rise above 1.degree.-2.degree. C./cm at 1200.degree. C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve that level of temperature uniformity, reliable real-time, multi-point temperature measurements for closed-loop temperature control are necessary.
Optical pyrometry is being widely used for measuring temperatures in RTP systems. Pyrometry exploits a general property of objects, namely, that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. Thus, by measuring the emitted radiation, the objects temperature can be determined. A pyrometer measures the emitted radiation intensity and performs the appropriate conversion to obtain temperature (T). The relationship between spectral emitted intensity and temperature depends on the spectral emissivity of the substrate and the ideal blackbody radiation-temperature relationship, given by Planck's law: ##EQU1## where C.sub.1 and C.sub.2 are known constants, X is the radiation wavelength of interest, and T is substrate temperature measured in .degree.K. According to an approximation known as Wein's distribution law, this expression can be rewritten as follows: ##EQU2## where K(.lambda.)=2C.sub.1 /.lambda..sup.5. This is a good approximation for temperatures below about 2700.degree. C.
The spectral emissivity .epsilon.(.lambda.,T) of an object is the ratio of its emitted spectral intensity I (.lambda.,T) to that of a blackbody at the same temperature I.sub.b (.lambda.,T). That is, ##EQU3## Since C.sub.1 and C.sub.2 are known constants, under ideal conditions, the temperature of the wafer can be accurately determined if .epsilon.(.lambda.,T) is known.
However, despite its widespread use in the semiconductor industry, optical pyrometry still suffers from limitations due to an inability to accurately measure the emissivity of the substrate. Moreover, even if the emissivity of the substrate is known at a given temperature, it changes with temperature. The changes are usually not accurately measurable and thus they introduce an unknown error into the temperature measurements. Errors on the order of 10.degree. C. or more are not uncommon.
The emissivity of a substrate depends on many factors, including the characteristics of the wafer itself (e.g. temperature, surface roughness, doping level of various impurities, material composition and thickness of surface layers), the characteristics of the process chamber, and the process history of the wafer. Therefore, a priori estimation of substrate emissivity cannot provide a general purpose pyrometric temperature measurement capability.
Techniques for reducing the effect of changes in wafer emissivity on temperature measurements are known. One such technique involves placing a thermal reflector near the back surface of a target substrate to form a reflecting cavity which causes thermal radiation from the substrate to be reflected back to the substrate. A light pipe, which is inserted through the reflector into the cavity, samples radiation from the reflecting cavity and delivers the sampled light to a pyrometer. Assuming an ideal reflector, it can be shown mathematically that because all of the thermal radiation emitted from the substrate is reflected back onto the substrate, the reflecting cavity acts like an ideal black body. That is, the intensity of the thermal radiation within the reflecting cavity will not be a function of the emissivity of the surface of the substrate. Stated differently, in the ideal case the reflecting cavity increases the effective emissivity of the substrate to a value equal to one. However, because the reflector will be less than perfect, the effective emissivity of the substrate will be higher than the emissivity of the wafer but less than one. Nevertheless, variations in the actual emissivity of the wafer will have considerably less impact on the measured temperature.
Though the above-mentioned temperature measurement schemes have achieved acceptable results, there is still considerable room for improvement.